Tonight a gripping crime story that involves a church youth group on a cross-country trip and ends up it one man dead and another a pastor now about to be tried for murder. Exactly what happened and how remains in dispute. But even prosecutors appear to have changed their minds about the evidence. It's part of our series "Crime & punishment." Everything I did that night was in self defense. Reporter: He is a California youth pastor. A beloved community leader. A married father of two. Now, charged with murder. But, was itself defense? Oh, I was protecting. Reporter: For 35-year-old, a brief St in Las Vegas, on a mission trip cross-country turned his life upside down. I just scant believe thcan't believe this is happening. It was one year ago that they stopped at this sports bar for dinner. They were in the parking lot when links Ellington, a stranger approached the group and began to aggressively harass them. He said, hey, girlfriend, you want to F with me? And rob immediately was like, oh, my gosh, no. Backed up. I have kids here. No. And the man said well I am coming to F with you. We came in contact with him. I felt all of us were in danger. Cox intervened to protect them and push Ellington away. Other witnesses who were there back up cox's account including intern Kelly lane. Rob could even get his arms around him to push him back. Unfortunately when he was pushing him back, they tripped and fell. Reporter: His head hit the pavement. The police arrived on the scene. And cox says it was then he found out Ellingson was injured. We came around him and parade for him. He eventually fell into a coma. He couldn't swallow anymore or tell me he loved him. Reporter: Initially it seemed the police believe the accounts. The local D.A. Wrote a letter at that time informing cox and his parishioners that no charges would be filed. Then, six months later, in December of last year, Ellingson died and everything changed. I have never had such heartbreak that I had that day. It was at that point that authorities took a new look at the case. And last week, nearly a year after the incident, the police showed up at cox's church doorstep with an arrest warrant for murder. And three police officers came in, we need to talk to you. And they arrested me. And I said what is going on? They said you have a warrant for murder. The coroner ruled the death a homicide. Finding that blunt head trauma from assault was a% significant cause of elellingson's death. And in court record, there was premeditation, deliberation and that he punched Ellingson in the face. Why was he suddenly being charged now. Prosecutors focus on what day allege was a diskreps acrepancy. He said he punched Ellingson. In written testimony that followed. Cox claimed that he grabbed him by the waist and they both fell to the ground. He was released on bail this week and emerged holding a bible. I could just explode right now. I want to help my children and my wife. That's all I was doing was protecting and -- I love you. I just can't wait to see them. How many witnesses do I have? Today cox appeared in court and pled not guilty. I am focused on my case, my innocence, my strength is my family. Reporter: Former prosecutor Nancy grace says the initial letter from the D.A. Will be crucial. The elected district attorney writes the family, there are no charges, you're fro to go. Reporter: But the D.A. Did say in that letter that they retain the right to file these charges at a later time if facts and circumstances warranted it. A camera did capture much of the altercation. According to the police report, the fight went out of view for the crucial moment when the police believe cox hit Ellingson. How do you get a conviction against a youth pastor with his youth group? Traveling around, the witnesses are saying was, was, not the aggressor. The only way I think there could be a conviction if the medical examiner report shows that there was blunt trauma on more than one side of the head. If they can show that there are enough traumatic injuries to Ellingson, the victim's head that could not have come from a fall alone, they may have a shot. Reporter: Member of pastor cox's youth group are expected to testify in his defense. He has a slew of eyewitnesses. It will be the eyewitnesses versus the prosecutor in this case. I think the youth pastor is going to carry the day. Reporter: ABC news reached out to the Clark county district attorney's office, but they would not comment. The entire incident has left the Ellingson family devastated. There are definitely pictures that -- that I don't want to keep in my head. I really do believe this is a very weak case. Reporter: Nancy grace thinks this is a very weak prosecution case. It is going to bile dooil down to this, when did Ellingson, by all accounts, the initial aggressor, when did he turn into the victim in this scenario, or did he? I am innocent. I acted to pro tkt test myself, my family and my students. And never intended to hurt anybody. Reporter: Cox is scheduled back in court July 2nd.

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